Guard drop technique for ink jet systems

ABSTRACT

BECAUSE OF THE FAIRLY CLOSE SPACING BETWEEN INK DROPS WHICH IS USED IN THE PROCESS INK DROP PRINTING, THERE ARISES A PHENOMENON OF REPULSION BETWEEN ADJACENT DROPS WHICH HAVE SIMILAR CHARGES ON THEM, WHICH CAUSES FAULTY PRINTING. THIS IS OVERCOME BY PROVIDING GUARD DROPS BETWEEN THE CHARGED DROPS WHICH ACT AS A SHIELD TO MINIMIZE THE ADVERSE AFFECTS OF DROP CHARGE REPULSION.

Feb. 9, 1971 v QFF 3,562,757

GUARD DROP TECHNIQUE FOR INK JET SYSTEMS:-

Filed Feb. 28, 1968 SOURCE 18 OF 16 SYNC.

\ ELECTRO- SIGNALS 1O MECHANICAL INK 12 TRANSDUCER v "UNDER vIDEO RESSURE SIGNAL SOURCE PRIOR ART- FIELD 34 POTENTIAL SOURCE CATCH R TO ELECTROMECHANICAL TRANSDUCER 1 II 56 58 O RC I r OF FREQUENCY GATE SYNC. DIvIDER To S'GNALS CHARGING SOURCE RING OF VIDEO- F G. 3

SIGNALS 52 NOZZLE SIGNAL GUARD CHARGED DROP DRQPG, T

42- Q Q Q CHARGED GUARD 4 I 46 48 50 DROP DROP v 54 INVDNTOR;

VINCENT E BISCHOFF VIDEO SIGNAL W PW ATTORNEYS United States Patent O 3,562,757 GUARD DROP TECHNIQUE FOR INK JET SYSTEMS Vincent E. Bischotf, River Grove, Ill., assignor to A. B. Dick Company, Chicago, 11]., a corporation of Illinois Filed Feb. 28, 1968, Ser. No. 709,004 Int. Cl. G01d 15/18 US. Cl. 3461 Claims ABSTRACT OF THE DISCLOSURE Because of the fairly close spacing between ink drops which is used in the process of ink drop printing, there arises a phenomenon of repulsion between adjacent drops which have similar charges on them, which causes faulty printing. This is overcome by providing guard drops between the charged drops which act as a shield to minitnize the adverse affects of drop charge repulsion.

BACKGROUND OF THE INVENTION This invention relates to apparatus for printing video signal information using ink drops deposited on a writing medium, and more particularly to improvements therein.

Apparatus has been developed for making a record on a writing medium of the information represented by video signals by generating a stream of ink drops, directing these ink drops toward the writing medium, and then, deflecting the ink drops in response to the video signal, in a manner so that when the ink drops do reach the writing medium, they provide a representation of the information contained in the video signals. The general apparatus employed for producing the ink drops consists of an ink reservoir in which there is ink under pressure. The ink reservoir feeds a pipe which is connected to a nozzle. An electromechanical transducer is employed to Vibrate the pipe and the nozzle at some suitable high frequency which causes the ink to be ejected from the nozzle in a stream which shortly thereafter breaks into individual drops.

In the region just before the stream breaks into drops there is placed a charging tunnel through which the stream is projected, which serves the function of applying video signals to the individual drops. Downstream of the tunnel there is provided a pair of deflection plates which have a fixed potential thereacross. The electric field which is created between the plates acts on the charged drops causing them to be deflected in an amount determined by the amplitude of the charge on the drops. Downstream of the deflection plates is usually a nozzle or trough for catching any drops which do not have any charge and transferring them to a waste reservoir. There is also positioned the writing medium which is to receive the deflected ink drops, which thereby form the images representative of the video signals. The writing medium is usually moved in synchronism with the application of video signals to the drops.

It has been found preferable in printing of the type briefly described above to maintain a center to center drop spacing of about five times the diameter of the orifice in the nozzle which forms the drops. Under these conditions, the effects of electric repulsion between adjacent drops with similar charges on them is just tolerable. However, in order to maintain such a drop spacing, a high ink supply pressure must be used. As a result, the drops 3,562,757 Patented Feb. 9, 1971 are quite large and the deflection sensitivity is low, due to the concomitant increase in drop velocity.

OBJECTS AND SUMMARY OF THE INVENTION An object of'this invention is the provision of an ink charging technique whereby the effects of electric charge repulsion between adjacent drops is minimized.

Another object of this invention is the provision of an ink drop charging technique which enables a reduction in the ink supply pressure while minimizing the adverse effects of electric charge repulsion between adjacent ink drops.

Still another object of this invention is the provision of a novel and improved ink drop charging and printing system.

These and other objects of the invention are achieved by an arrangement wherein the video signal being applied to the ink drops is gated in a manner to insure that each drop which is charged by a video signal is preceded by and is followed by one or more drops which have no charge. These are called guard drops. These guard drops act as a shield between the charge drops. They provide a considerable reduction in the electric repulsion between charge drops.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic arrangement for ink drop recording in accordance with the prior art, which is shown for purposes of explanation.

FIG. 2 is a block schematic diagram of a circuit arrangement employed for charging drops with video signals so that guard drops may be formed.

FIG, 3 is a wave form diagram representing the nozzle signal and video signals which are obtained when this invention is employed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is provided in order to afford a better understanding of this invention, and it shows schematically the known ink drop recording apparatus. This apparatus comprises an ink reservoir 10 in which the ink is placed under pressure. Tubing 12 connects the ink reservoir to a terminating nozzle 14. An electro-mechanical transducer 16 is usually employed for vibrating the tubing and more specifically the nozzle, in response to the output signals from a source of sync signals 18. Near the tip of the nozzle 14, the ink is emitted as a solid stream 20 which, however, shortly thereafter breaks into drops, such as 22. A charging tunnel 26, consisting of a conductive tube, shown open for the sake of illustration, is positioned at the location at which a stream of ink begins to break up into drops. The video signal source 24 has its output connected to the conductive tubing 12 and to the charging tunnel 26, as a result of which each drop 22 will assume a charge whose amplitude is proportional to the video signal applied between the charging tunnel 26 and the tubing 12 at the time that the drop is breaking away from the ink stream within the tubing 26.

The charged drops then continue in a path toward the moving paper 36. However, before they reach the paper, they pass between two spaced, parallel plate electrodes 30. 32, to which a fixed potential is applied from a field potential source 34. The charged drops are caused to enter an electric field established between the electrodes 30, 32, as a result of which each drop is deflected from the path it would take in the absence of the electric field, an amount determined by the charge thereon. Trough 38 is positioned to catch drops which do not have a charge, and conducts these drops to a waste catcher 40. Accordingly, as the paper 36 is continuously moved, a line is drawn thereon whose shape or curvature is determined by the signals from the video signal source.

Alpha numeric characters may be written by this apparatus as well as a line or curves. In effect therefore, the recording on paper is representative of the information conveyed by the video signals.

In order to accurately reproduce the information contained in the signals received from the video source, it is necessary that a charged drop which enters the fixed electric field be undeflected or affected by factors other than the interaction of the fixed field and the drop charge. However, one of the factors that has been found to adversely affect drop deflection is the effect on a drop of the charge on a preceding Or a succeedin drop. It will be appreciated that since the charges vary with the amplitude of the video signal which is applied to the charging tunnel at the time that a drop is formed, and since each charged drop in and of itself establishes an electric field Whose strength is determined by the amplitude of that charge, the location at which a given charged drop is placed on the paper will not necessarily be repeatable even though succeeding drops receive the identical charge thereon, if the drops succeeding and preceding each uniformly charged drop have varying charges.

It was found that in order to establish a situation where the effects of electric repulsion between adjacent drops could be tolerated, a large spacing was required between drops. However, in order to maintain such a large drop spacing, a high ink supply pressure has to be used, and as a result, the drops are quite large and have a high drop velocity. This causes the system to have a poor drop deflection sensitivity.

Instead of maintaining a large drop spacing, which effectively reduces the strength of the field effect of one drop upon another by reason of the increased distance between drops, in accordance with this invention, guard drops are provided between the charged drops. These guard drops which have no charge, act as a shield between the charged drops. As a result, there is a considerable reduction. of the drop charge repulsion phenomenon. Accordingly, a much lower ink pressure can be used and therefore smaller ink drops are formed having a lower velocity. This provides a greater deflection sensitivity with a minimum pattern distortion due to the elimination of the drop charge repulsion problem. In addition, an unlooked for benefit is that the aerodynamics of the system is improved. Each drop leaves in its wake a region of turbulence which causes some uncertainty in the predictable path of a following drop which enters the region of turbulence. Since, by using guard drops, effectively when the region of the electric field is reached, the guard drops are removed from between the charged drops which are being deflected, the spacing between the remaining charged drops is increased and therefore the effects of turbulence are effectively eliminated.

FIG. 2 is a wave form drawing illustrating an arrangement of drops and video signals in accordance with this invention. The drops are represented by the circles 42, 44, 46, 48, and 50. The wave form 52 represents the wave form of the synchronizing signal which is applied to vibrate the nozzle. As shown, there is a drop formed for each cycle of the wave form. In accordance with this invention, the video signal is gated in such a way as to insure that each drop, which is charged by a video signal is preceded by and followed by one or more drops which have no charge. Thus, the wave form 54, which represents the video signal Wave form is present only when the alternative drops 42, 46 and 50 are formed. The intervening drops 44 and 48 have no charge and act as guard drops.

FIG. 3 is a block schematic diagram of an arrangement whereby guard drops are provided. Effectively all that is required to modify the arrangement shown in FIG. 1 is the insertion of a frequency divider or counter circuit 56 and a gate 58. To divide by two, whereby alternate drop charging occurs, frequency divider 56 is a flip-flop circuit. The flip-flop circuit is driven from its set to its reset state in response to the signals from the source 18. One of the flip-flop outputs is used to enable the gate 58 to pass signals received from the source of video signals 18. Since, as may be seen by FIG. 2, the nozzle forms a drop for every cycle of the signals received from the source of sync signals 18, a video signal will be passed by the gate 58 upon the occurrence of every other drop which is formed. There is very little loss in information by gating the video signal in the manner shown, since a large portion of the video information in the signals is redundant and the arrangement shown merely serves to eliminate some of the redundancy which normally occurs.

It should be appreciated that a frequency divider which divides by three secures two guard drops and thus the shielding effect is better. Thus, the use of a flip-flop as a frequency divider should be construed as exemplary and not as a limitation on the invention.

Accordingly, there has been described and shown herein an improved ink drop writing system wherein a system can be operated with a lower ink supply pressure a greater deflection sensitivity, a lower ink drop velocity with less ink splatter occurring, smaller ink drops may be used with larger charge signals and a better aerodynamic behavior.

What is claimed is:

1. An improved method of ink drop writing in a system wherein video signals are applied from a source to an ink drop charging means for applying a charge to each ink drop in a stream passing through said ink drop charging means, said improvement consisting of separating each drop in a stream which has a charge by at least one drop in said stream which has no charge.

2. The method as recited in claim 1 wherein said step of separating each drop in a stream which has a charge from another drop by a drop in said stream which has no charge includes applying signals for charging ink drops to said ink drop charging means solely from said video signal source at spaced intervals for charging at most every other drop to provide drops with no charge between drops with charge.

3. In an ink drop writing system wherein ink drops are formed by vibrating tubing having a nozzle, and there is provided a video signal source, meansfor char-ging drops in a stream of drops in response ,to signals from said video signal source, and an electric field through which said charged ink drops are thereafter passed to be deflected in accordance with the charge on each drop, and a writing medium on which said drops are deposited, the improvement comprising:

means for providing drops with no charge between drops with charge, including means for applying signals to said means for charging drops in a stream solely from said video signal source at intervals for charging non-adjacent drops in a stream of drops.

4. In an ink drop writing system as recited in claim 3 wherein said means for applying signals from said video signal source to means for charging drops includes gate means for applying video signals from said source to said means for charging drops, and

means for enabling said gate means to pass video sig nals only during an interval wherein alternate drops in said stream of drops are available for being charged by said means for charging drops.

5. In an ink drop writing system as recited in claim 3 wherein said ink drop writing system includes a source of sync signals,

means for applying sync signals from said source to References Cited said video signal source to sync charge said video UNITED STATES PATENTS signals therewith, and

3,369,252 2/1968 Adams 346--75 means for vlbrating said vlbratlng nozzle responsive to said sync signals, said means for enabling said 5 FOREIGN PAPTFTNTS gate means includes dividing means for dividing the 1,042,585 9/ 1966 Great Brltaln 346-75 frequency of sync signals from said source, and JOSEPH H ART ARY, Primary Examingr means for applying the output of said dividing means to said gate means to enable said gate means re- 10 sponsive thereto to pass said video signals. 1786.6; 3173; 34675 

